(1) Field of the Invention
This invention relates to recorders such as those adapted for data logging applications in which data is recorded, together with a code indicative of the time at which the data is received. In particular, this invention is directed to a recorder in which such a time code is multiplexed onto the data and is recorded therewith, being distinguishable therefrom by the frequency components present in the code.
(2) Description of the Prior Art
A variety of recorders in which certain data such as a time code is multiplexed onto other data and the combined signals recorded onto a record medium have previously been known. For example, as early as 1934, it was disclosed in U.S. Pat. No. 1,960,094 (Thurm) that a number of separate records could be magnetically superimposed onto a magnetic recording ribbon by recording each of the records at a different speed. The separate records are there disclosed to be recovered by passing the playback signal through appropriate filters corresponding to the desired reproduced frequencies.
More recently, the specific need to record time related information along with other data and at a frequency outside the frequency spectrum of the other data has been particularly recognized. In U.S. Pat. No. 3,188,615 (Wilcox), there is disclosed an instrumentation recorder in which wide band width data is recorded on a transverse track system together with a standard frequency signal, which is outside the frequency spectrum of the wide band data. In the recorder there disclosed, the data is frequency modulated within a selected range of frequencies and the standard frequency signal which is outside that range of frequencies is added linearly thereto.
U.S. Pat. No. 3,591,731 (Stancil) discloses a recorded in which real time information is recorded simultaneously with other information, rather than merely providing a series of clock or sync pulses which are only referenced at the beginning or end of the recording. In that patent, the time code is said to be generated from sync motors which provide a 60 Hz output voltage proportional to the angle of a clock hand which is outside the frequency range of the other data, thus enabling the use of a bandpass network to reconstruct the time code. Such an analog system is incapable of providing a digital time code and requires a number of channels for each digit of time information.
U.S. Pat. No. 3,732,364 (Terada) discloses a system for multiplexing video and digital signals, in which the digital signal, typically used to represent a file address, is caused to create bursts of a sinusoidal signal having a base frequency outside the video range. The modulated sine wave is then superimposed on the horizontal sync pulse of the video signal. Preferably, digital "1"s and "0"s are associated with bursts of different sinusoidal frequencies.
U.S. Pat. No. 3,945,035 (Goldman) discloses a tape recorder in which a plurality of recording heads are positioned in tandem with each other, spaced apart a distance corresponding to the period of a desired frequency as recorded on the tape. The desired frequency signal is applied simultaneously to all the heads such that the recorded amplitude of the desired signal component is increased over that of all other frequencies. Such a system avoids the need to selectively filter the signals obtained upon playback to recover time related information stored in frequency bands outside that utilized by other information; however, the added complexity in which a number of heads and means for adjusting the spacing therebetween are required is undesirable for high reliability applications.